Scada System

SCADA stands for Supervisory Control and Data Acquisition. SCADA refers to a system that collects data from various sensors at a factory, plant or in other remote locations and then sends this data to a central computer which then manages and controls the data. The feedback control loop passes through the remote terminal unit(RTU) or Prgrammable logic controller (PLC), while the SCADA system monitors the overall performance.For example:A PLC may control the flow of cooling water through part of an industrial process, but the SCADAsystem may allow operators to change the set points for the flow, and enable alarm conditions, such as loss of flow and high temperature, to be displayed and recorded.Most essentially the SCADA system consists of three fundamental components (i.e. Master station (MS) or Central Monitoring System (CMS), Communication link and RTU or PLC).

A Central Monitoring System (CMS), contained within the plant and one or more Remote Stations. The CMS houses the Control Server and the communications routers via a peer-to-peer network. The CMS collects and logs information gathered by the remote stations and generates necessary actions for events detected. A remote station consists of either a Remote Terminal Unit (RTU) or a Programmable Logic Controller (PLC) which controls actuators and monitors sensors.

Typically, remote stations have the added capability to be interfaced by field operators via hand held devices to perform diagnostic and repair operations. The communications network is the medium for transporting information between remote stations and the MS. This is performed using telephone line, cable, or radio frequency. If the remote site is too isolated to be reached via direct radio signal, a radio repeater is used to link the site.

Simulation tools used to aid the operator in preventing disturbances.Simulation is used when the system which is to be analyse or design is complex to use analytical techniques (like aerodynamics).Simulation models can be descriptive and physical.Simulation models that are formulated called descriptive models.Physical models mostly are time-discrete showing the act and interact of entities of a system. These models can be built after many steps involving computer programs. The state of the system changes as a consequence of events in the system . These events are considered to occur at discrete times; models are called discrete event simulation models.

Simulation languages Most well-known simulation languages are GASP IV, GPSS and SIMSCRIPT II.1

The process can beIndustrial:include those of manufacturing, production, power generation, fabrication, and refining,

Infrastructure : include water treatment and distribution, wastewater collection and treatment, oil and gas pipelines, electrical power transmission and distribution, and large communication systems.

Facility based : including buildings, airports, ships, and space stations. They monitor and control HVAC, access, and energy consumption. HVAC "heating, ventilating, and air conditioning" is sometimes referred to as climate control

Little More About SCADA

A SCADA system manages electricity distribution by collecting data from, and issuing commands to, geographically remote field control stations from a centralized location.

In the Oil and Gas industry, refining and processing facilities use distributed control systems (DCS) while holding facilities and distribution systems utilize SCADA technology.

The water supply infrastructure utilizes DCS for the processing operations but use SCADA technology for distribution operations. A waste treatment plant runs similar to that of the water supply infrastructure.

Suppose you have a simple electrical circuit consisting of a switch and a light. Like this

This circuit allows an operator to watch the light ( ) and know whether the switch is open or closed. The switch may indicate that a motor is running or stopped, or whether a door is open or closed, or even whether there has been a fault or the equipment is working.So far there is nothing special about this. But now imagine that the switch and the lamp were 100 kilometres apart. Obviously we couldn't have an electrical circuit this large, and it would now be a problem involving communications equipment Now complicate the problem a bit further. Imagine we had 2000 such circuits. We could not afford 2000 communications circuits. However someone found that we could use one communications circuit by sharing it. First we send the status (open/closed or 0/1) of the first circuit. Then we send the status of the second circuit, and so on. We need to indicate which circuit the status applies to when we send the data.The poor operator at the other end still has a problem.He has to watch all 2000 circuits. To simplify his task, we could use a computer. The computer would monitor all circuits, and tell the operator when he needs to look at a particular circuit. The computer will be told what the normal state of the circuit is and what state is an "alarm". It monitors all circuits, and informs the operator when any circuit goes into alarm.Simply called SCADA.

Breaker Mechanism

CT

VT

Transducer

RTU

Station computerCentral control

(SCADA)

Protective relay

Mimic board

Analog meters

Monitoring & Control of local system using RTU:

Automatic functions for local control performed by regulators, as well as sequence control performed by relays are integrated as software functions in the RTU.

One of key processes of SCADA is the ability to monitor an entire system in real time (run with relatively little human intervention). This is facilitated by data acquisitions including meter reading, checking statuses of sensors, etc.

The SCADA software is a graphical package using a Window NT Operation System.  The system, manufactured by Power Measurement Ltd (PML), provides the programming necessary to incorporate actual AutoCAD campus maps and diagrams, and to display real time information on top of the graphic background.

TECHNICAL FUNCTIONS OF SCADA:

The SCADA system provides the following functions:Data acquisition:The basic information of the power system collected is called the Data Acquisition. The data is collected by means of CTs, PTs and transducers. It provides the telemetry measurement and status indication to the operator. Supervisory control:It enables the operator to remotely control the devices. For example open and close of the circuit breaker.Tagging:It prevents the device from unauthorized operation. Means it authorizes the device to perform the specific operation.Alarms:It informs the operator about the unnecessary events and undesired conditions.Logging (Recording):It logs all the operating entry, all alarms and other information. In other words it keeps the record of all the events.Load shading:It provides both the automatic and manual control tripping of load during the emergency.Trending:It plots the measurement on the selected time scale.

ESSENTIAL FEATURES OF SCADA:The simple layout of the SCADA system with its essential features is shown below in the fig:

The SCADA system enables the operator to attain the complete knowledge of the system in a single room by means of display.Almost all the SCADA systems are computer based (Digital computer). This computer is located in the master unit. The master unit is the heart of the SCADA system, it comprises many of the i/p and o/p equipments to receive and send the control message from and to the RTU.

All the data of operations of RTU are transmitted to the master control unit and after collecting the information the data are feed back to the RTU. Also master unit consists of several modems which are used to convert the digital into analog or analog into digital message depending upon the requirement. The received information of the master unit is displayed on VDU and then is printed for permanent record.

In addition the SCADA system also comprises some more peripheral equipment such as control console, VDU, Alarms, Printers, D/A converter and Recording instruments. Visual Display Unit (VDU) replaces mimic board to represent one line diagram, tabular display, bar charts, curves and event lists and used for entering commands to system.Modern system includes the color display which is used to distinguish b/w the different voltage levels. Also different colors differentiate the operator to understand the open and close of CB, also the flashing indication can be made which determines the change of the state of any device. The audible alarms can be used to alert the operator from the fault or condition. The printers are used to have the permanent records of the events. The D/A converter are used to convert the digital information into the analog information, and then the information is supplied to the indicating or the recording instruments. The recording instruments are used to store the data of each remote station unit.

FUNCTIONAL DIAGRAM OF SCADA:The functional diagram of SCADA consists of several parts such as Data base, Data acquisition, Main machine interface and application program etc, are shown below in the fig:

Data acquisition is the function of the SCADA which is required to perform the following functions:

• It is used to read the power system data from 100-250 RTU into the control center.

• It detects and handle the error conditions of RTU and communication link as well as.

• It can be used to check the limits such as high/low operating limit, thermal limits, and safe handling limits.

• It can be used to convert the analog data into the binary data.• It stores only the error free data in the data b/c erroneous data causes the

failure of program.• It can interface with the data base manager, which generated the data

address where data can be stored in data base.• It completes all above tasks in less than 2 seconds.Typically data acquisition is implemented by the master station, software, and

computer. The software consists of hundreds of different programs; at the same time these program needs access to describe the structure and state of the power system. Therefore the data base plays an important and central role in control system design. Thus the SCADA system requires an efficient and effective data base, data acquisition and main machine interface.

Data base as the name implies is base for all the functions (i.e. data acquisition, main machine interface and application program). The primary object of the data base is to provide a well defined processing for several functions. Data base is repertory for all power system dynamic data, common data, user program and manually entered variables from operator console and network data such as historical files and record of data. It physically resides within the computer system. The data base in the SCADA system provides the following advantages:

• It provides organized structure of data expressed in file.• It stores the data with very little space require. Also it provides the flexibility in the

selection of storage medium.• It provides the discrimination b/w data and program or b/w two programs.• It provides a well defined expansion of software system.

A SCADA system includes a user interface, usually called Human Machine Interface (HMI). The HMI of a SCADA system is where data is processed and presented to be viewed and monitored by a human operator. This interface usually includes controls where the individual can interface with the SCADA system.

Usually RTU's or PLC's will run a pre programmed process, but monitoring each of them individually can be difficult, because they are spread out over the system, HMI's are an easy way to standardize the facilitation of monitoring multiple RTU's or PLC's (programmable logic controllers).

HMI's can also be linked to a database, which can use data gathered from PLC's or RTU's to provide graphs on trends, logistic info, schematics for a specific sensor or machine or even make troubleshooting guides accessible.

PERFORMANCE MONITORING:

The performance monitoring is used to enhance the operations availability of SCADA or control center systems. The basis requirement for the control center is the availability of the SCADA and its communication systems as well as RTU. All of these equipments must be greater than the rated capacity of the power system network. The power system must remains in the operation even at the time of the electrical faults, mechanical failure, and other environmental conditions. The basic philosophy of performance monitoring is that if any single fault occurs with the SCADA system, then the SCADA system should be able to withstand against that fault and operates without any disturbance. For that purpose several facilities such as fail over equipments, backup hardware and device are provided with the computer to bypass the faulty program in order to recover the operational status of the SCADA system.The availability is increased by the equipments redundancy. The software availability is increased by including the error detection, error handling, and fail over recovery and automatic restart facility. The performance monitoring nowadays is used in all SCADA ad other control centers b/c it provides the whole software availability and control of hardware.

OPERATING SYSTEM:

The operating system is required to provide a real time priority and oriented process environment for computer. The operating system provides the following facilities:

Multi-computing, multi-programming environment for program execution. Also multi-processor are desirable for availability point of view.

(ii) Timer facilities.(iii) Interrupt handling(iv) Error detection and error handling(v) Input and output handling(vi) Recovery and automatic restart facility(vii) On line file management (viii) Language processor

LAYOUT OF VDU DISPLAY:

The VDU display is used in SCADA to observe different states of the system. It enables the operator to find out the desired information in fast manner. The VDU display consists of two kinds of information (i.e. General information and specific information).The general information as the name implies is generally used for all display and is thus always shown on the screen. The specific information is used to any specified device, it is only used when is desired. The general information will of course different for different systems. But the most general layout of the VDU display is shown below in the fig:

•The display identification normally shows substation name and type of information.•Page no block shows the page numbers, if the pages are more than one then X (Y) form is used.•System block shows the present part of the system on which the operator is working.•Actual time block indicates the time of day.•The latest unacknowledged alarm block shows the alarm alert situation.•Dynamic functional keys comprises several keys, each key has got its own work such as display paging and object selection etc.•The dialog field indicates the error message to the operator or it contains any other message entered from the operator

PRESENTATION (CRT DISPLAY):The CRT stands for Cathode Ray Tube is available in the control room; it provides the information about the i/p quantities to the operator. The information about any equipment can be displayed from the screen by simply pressing the button on the control desk. CRT display is located in control room of generation station, sub-stations and control center. The CRT display includes:Alpha-numeric Displays: These displays show the reading and name of the measured parameter.Single Parameter Displays: These displays show the numerical data for certain parameter in tabular form.

Mimic Displays: These display show the single line diagram of CB, Isolator and power flow in the network.

Display with threshold blackout: These displays show the threshold values of the quantity. Threshold means on the border.

Graphical Display: These displays show the graph of quantities.Histogram Display: These displays show the graphical representation of a

distribution of quantity.Pictorial Displays: These displays can be used as small mimic display. It shows the

line diagram of a plant indicating positions of equipments.Analogue Displays: These displays show the continuously varying parameters.Alarm Displays: These displays show the control board for the attention of the

operator.OPERATING SYSTEM:

The operating system is required to provide a real time priority and oriented process environment for computer. The operating system provides the following facilities:

(i) Multi-computing, multi-programming environment for program execution. Also multi-processor are desirable for availability point of view.

(ii) Timer facilities.(iii) Interrupt handling(iv) Error detection and error handling(v) Input and output handling(vi) Recovery and automatic restart facility(vii) On line file management (viii) Language processor

KEYBOARD DESIGN:The keyboard is the device by which the operator enters the command to the computer. The keyboard contains the following keys:

Keys for display and substation selection:Subsystems and StationsOne lines, tabular display, event display, curves etcFunction menu

Keys for function selection:ControlRegulation (Raise/Lower)Data entry and Cancel

Keys for function execution:Close/Start and Open/StopSelectEnd of entry

Keys for console assignment:- Primary/Backup- Multi VDU selection-Real time/Training/Study

Dynamic keys:These keys are used to show the display request or a function start-up on the display. The dynamic keys are used to guide the operator as the part of the description.

Alphanumeric and graphical symbols:These are used to display the alphanumeric and graphical symbols on the VDU.

Keys for display editing:Control keys for writing direction:Cursor positioning keys:

MAIN TASKS IN POWER SYSTEM OPERATION:The main tasks of power system can be divided into the following categories:1. Planning operations 2. Operation control3. Operating accounting and financial controlThe above tasks are performed by the National control center, Regional control center, District control center and Major substation control rooms.

1) Planning of operations:Planning means the formulating and preparing actions before put into the use. Planning is done for next hour, next day, next week, next month, and next year and for long range. Planning tasks includes: The national load center performs the following tasks:First of all load prediction followed by generator scheduling, spinning reserves, planning of reserves, planning of maintenance schedules, planning of energy resource, selection of energy resource, hydro-thermal generation co-ordination, planning of power exchange b/w two regions and planning of installation of HVDC lines. The Regional load center performs the following tasks:First of all load prediction followed by generator scheduling, planning of reserves, planning of maintenance schedules and selection of load scheduling. The District load center performs the following tasks: First of all short term planning in terms of regional level planning and then followed by planning of generation and reserves, and planning of load shedding in district. The Power station and sub-station control room performs the following tasks: Work planning for hourly, daily, weekly, monthly.

2) Operational control tasks:An electric supply corporation basically must meet the following aims:

-There must be continuity of the power supply for the consumer.-The network will be designed in such a manner so as to cover the ---

maximum consumers.-Maximum security of supply-Less possibility of the fault-Maximum efficiency of the plant-Consistency of the frequency-Consistency of the voltage-Supply power at low cost

The national load center performs the following tasks:-Accuracy of generation, and load and frequency-Power exchange under emergency case-System frequency controlThe Regional load center performs the following tasks:-Accuracy of generation, load and frequency and power exchange b/w

districts under emergency.The District load center performs the following tasks:-Accuracy and control of generation in district-Control of power station-Control of T/L and tie-lines

The power station and sub-station control room performs the following tasks:-Generation, start, stop function-Automatic restoration function-Control and protection functionsMaintenanceThe transmission sub-station performs the following tasks:-Switching operation-Protective functions-Voltage control and reactive power compensation-Data collection-Load shedding instructions.

3) Operating accounting and financial control:The operating accounting deals with the data collection and evaluation and then preparation of the financial reports and billings. The task includes first of all the data collection regarding the power produced, and then billing on that power interchange for adjacent station, then estimate the performance of the power station, then estimate the price of the power available for the consumer.

Control Components of an Industrial Control

System (ICS)

•The key control components of an ICS, including the control loop, the HMI, and remote diagnostics and maintenance utilities, are shown in Figure .

•A control loop consists of sensors for measurement, control hardware, process actuators, and communication of measurement variables. Measurement variables are transmitted to the controller from the process sensors. The controller interprets the signals and generates corresponding control signals that it transmits to the process actuators. Process changes result in new sensor signals, identifying the state of the process, to again be transmitted to the controller.

•The HMI allows a control engineer or operator to configure set points, control algorithms and parameters in the controller. The HMI also provides displays of process status information, including alarms and other means of notifying the operator of malfunctions. Diagnostic and maintenance tools often made available via modem and Internet enabled interfaces, allow control engineers, operators and vendors to monitor and change controller, actuator, and sensor properties from remote locations.

•A typical industrial system contains a large number of control loops, HMIs and remote diagnostics and maintenance tools built on an array of network protocols. Supervisory level loops and lower level loops operate continuously over the duration of a process at cycle times ranging on the order of milliseconds to minutes.

See description in Annex. A

Following Figure provides a generic SCADA Network Architecture that provides for an electric power example and an oil & gas/water example.

may consist of line graphics and schematic symbols to represent process elements, or may consist

of digital photographs of the process equipment overlain with animated symbols.

Annex. A• Application Server – A computer responsible for hosting applications to user

workstations.• Backup Domain Controller – Backup to the Primary Domain Controller.• Control Server – A server hosts the supervisory control system, typically a• commercially available application for DCS or SCADA systems, and communicates data• between the Peer-to-Peer network and the LAN.• Data – A repository of information that usually holds plant wide information including

process data, recipes, personnel data and financial data.• Distributed Control System (DCS) – A supervisory control system typically controls• and monitors set points to sub-controllers distributed geographically throughout a factory.• Distributed Plant – A geographically distributed factory that is accessible through the

Internet by an enterprise.• Domain Controller - A Windows NT server responsible for managing domain information,

such as login IDs and passwords.• Enterprise – A business venture or company that encompasses one or more factories.• Enterprise Resource Planning (ERP) System –A system that integrates enterprise-wide

information including human resources, financials, manufacturing, and distribution as well as connect the organization to its customers and suppliers.

• Fieldbus - A category of network that links sensors and other devices to a PC or PLC based controller. Use of fieldbus technologies eliminates the need of point-to-point wiring between the controller and each device. A protocol is used to define messages over the fieldbus network with each message identifying a particular sensor on the network.

Firewall – A device on a communications network that can be programmed to filter information based on the information content, source or destination.Human Machine Interface (HMI) – The hardware or software through which an operator interacts with a controller. An HMI can range from a physical control panel with buttons and indicator lights to an industrial PC with a color graphics display running dedicated HMI software.Internet – a system of linked networks that are worldwide in scope and facilitate data communication services. The Internet is currently a communications highway for millions of users.Input/Output (I/O) – Module relaying information sent to the processor from connected devices (input) and to the connected devices from the processor (output).Light Tower – A device containing series of indicator lights and an embedded controller used to indicate the state of a process based on an input signal.Local Area Network (LAN) – A network of computers that span a relatively small space. Each computer on the network is called a node, has its own hardware and runs its own programs, but can also access any other data or devices connected to the LAN.Printers, modems and other devices can also be separate nodes on a LAN.Modem – A device that allows a computer to communicate through a phone line.Management Information System (MIS) – A software system for accessing data from production resources and procedures required to collect, process, and distribute data for use in decision making.Manufacturing Execution System (MES) – Systems that use network computing to automate production control and process automation. By downloading “recipes” and work schedules and uploading production results, a MES bridges the gap between business and plant- floor or process-control systems.

OPC Client/Server – Mechanism for providing interoperability between disparate field devices, automation/control, and business systems.Peer-to-Peer network – A networking configuration where there is no server and computers connect with each other to share data. Each computer acts as both a client(information requestor) and a server (information provider).Pressure Sensor – A sensor system that produces an electrical signal related to the pressure acting on it by its surrounding medium.Primary Domain Controller – A Windows NT server responsible for managing domain information, such as login IDs and passwords.Printer – A device which converts digital data to human readable text on a paper medium.Proximity Sensor – A non-contact sensor with the ability to detect the presence of a target, within a specified range.Redundant Control Server – A backup to the control server that maintains the current state of the control server at all times.Remote Terminal Unit (RTU) – A computer with radio interfacing used in remote situations where communications via wire is unavailable. RTUs are usually used to communicate with remote field equipment. PLCs with radio communication capabilities are also used in place of RTUs.

Sensor - A device that senses or detects the value of a process variable and generates a signal related to the value. Additional transmitting hardware is required to convert the basic sensor signal to a standard transmission signal. Sensor is defined as the complete sensing and transmitting device.Supervisory Control and Data Acquisition System (SCADA) – Similar to aDistributed Control System with the exception of sub-control systems being geographically dispersed over large areas and accessed using Remote Terminal Servers.Temperature Sensor – A sensor system that produces an electrical signal related to its temperature and, as a consequence, senses the temperature of its surrounding medium.Workstation – A computer used for tasks such as programming, engineering, and design.Wide Area Network – A network that spans a larger area than a LAN. It consists of twoor more LANs connected to each other via telephone lines or some other means of connection.Wireless Device – A device that can connect to a manufacturing system via radio or infrared waves to typically collect/monitor data, and also in cases to modify control set points.